Conventionally, the surface of a moving web of paper or board is smoothed and made glossy in a multiroll calender comprising a plurality of rolls stacked in a calender frame so as to form a nip contact with each other. The roll stack comprises a top roll and a bottom roll with at least one intermediate roll located therebetween. The rolls of the stack are compressed against each other by the top and bottom rolls that act as the loading rolls or, simply, by the bottom roll to provide a sufficiently high linear nip force. In calendering, the web passes through the calender nips formed by the superimposed rolls.
The rolls of the calender stack are mounted rotatably in bearing blocks that are usually attached to roll mounts. The roll mounts themselves are slidably connected to vertical guides adapted to the calender frame. In a conventional supercalender, the roll mounts are additionally connected to vertical screw jack assemblies adapted to the calender frame. When the roll stack is open, the positioning of the roll mounts in the vertical direction is accomplished by means of the jack assemblies comprising threaded screw rods and nuts running thereon. As each one of the mounts of the roll bearings rest on these jack nuts, the entire weight of the set of rolls is supported on the screw rods when the roll stack is unloaded. Bearing blocks of roll stack and thereby the rolls mounted thereon can be moved vertically in regard to the mounts.
The roll set of a multiroll calender has a plurality of rolls in a superimposed disposition, whereby the linear load imposed on the nips by the weights of the rolls increases nip-by-nip from the top nip to bottom nip, whereby the linear load in the bottom nip is the maximum stress imposed by the calender on the web passing the calender. Hence, the calender must be designed based on the load-bearing ability of the bottom nip, whereby a substantial portion of the potential calendering capacity of the upper nips remains unused. Also the weights of the roll bearing blocks and auxiliary devices connected thereto cause distortion in the linear pressure profiles of the nips, particularly at the nip ends, thus deteriorating the quality of the calendered web.
One technique developed for equalizing the nip loading is the so-called variable-crown calender, wherein the weights of the intermediate rolls do not essentially contribute to the linear load in the nips. In calenders equipped with variable-crown roll, the intermediate rolls of the stack are provided with load-relieving devices such as hydraulic load-relief cylinders or pivotal links connected to the calender frame, by means of which arrangements the linear load imposed by the intermediate rolls and auxiliary devices connected thereto can be relieved, thus allowing the nips to be loaded mainly by the variable-crown top and bottom rolls or, alternatively, an external load imposed on said rolls. In a load-relief system for the intermediate rolls, the design factors to be taken into account are the deflection stiffness, mass, shape and material properties of each roll. The support forces to be imposed on the intermediate rolls are varied with the help of the load-relief means so that the roll set is equilibrated and brought to a desired state of crowning. Variable-crown calenders are described, among other things, in U.S. Pat. No. 5,438,920.